Sega Master System / Mark III / Game Gear
On early Master Systems (with either no built-in game, or any game except Missile Defense 3-D, Alex Kidd in Miracle World or Sonic The Hedgehog), there is a secret built-in Snail Maze game.
No built-in game: turn on your Sega Master System without a game plugged in. At the screen that says to turn off your system and insert a game, press ++.
Built in game (except Missile Defense 3-D, Alex Kidd in Miracle World or Sonic The Hedgehog): hold ++ and power on the system. Continue holding the buttons until the game begins.
See the main page for Snail Maze